Double-clad fiber lasers and amplifiers can be considered to be a revolution in the laser industry. The ability of the double-clad fiber to convert low-quality, low brightness pump radiation into high brightness, signal radiation is a key attribute contributing to their success. The brightness conversion is performed by creating a situation where pump radiation inserted into the cladding intersects and is absorbed into the core of the fiber laser as the pump radiation propagates down the cladding. The core of the fiber laser contains a rare-earth dopant responsible for laser action and the core absorbs a portion of the intersecting pump radiation that is in the cladding.
More particularly, double-clad fiber lasers have provided an efficient method for generating high optical powers, average and peak, at high efficiencies. The very nature of optical fibers is conducive to high power operation. Fibers have high specific volumes that contribute to higher heat transfer capabilities, and the waveguide nature of the fiber provides high modal overlap of the pump and signal energy resulting in high conversion efficiencies. Power levels of >1 kW are readily achievable with double-clad fiber lasers with electrical efficiencies of >30%. A major limiting factor in further power scaling is the inability for a laser to dissipate the large amounts of waste heat from the rare-earth doped fiber core in a fiber laser. Such waste heat is mostly generated nearest the pump input end (launch end) of a doped fiber and its cladding in a double clad fiber laser, and the least waste heat is generated nearest the output end of the fiber.
A fiber laser can be end pumped or side pumped. In end pumping, the light from one or more pump lasers is fired into the end of the fiber laser into its core and into its cladding layer. In side-pumping, pump light is coupled into the side of the fiber; actually, it is fed into a coupler that couples it into the outer cladding core. This is different from side-pumping a laser rod, where the light comes in orthogonally to the axis.
The problem is that the doped core of the fiber laser is too small to focus the output of one or more pump diode lasers into it. To get around this problem, the diode pump laser beam is inserted into both the end of the core and into the end of the much-larger cladding layer around the core. This is called “core pumping” and “cladding pumping”. To contain the pump laser beam inside the fiber laser, the core and cladding layer of the laser are clad with an outer sheath. This way, the pump beam bounces around inside the cladding around the core as the pump beam propagates along the fiber. Every time the pump beam crosses the core, some of the pump light energy is absorbed into the core.
Fiber lasers can be quite long and waste heat generated therein is distributed over the length of the fiber which helps protect the fiber from getting so hot that it breaks. However, the waste heat distribution is not uniform along the length of the fiber because most of the heat is generated near the launching end of the fiber laser. Thus, the waste heat is not optimally distributed and eliminated along the length of the fiber to maximize protection against heat damage to the fiber laser. In addition, materials change their optical properties as they heat up and that affects the quality of the laser beam generated in the core of the fiber laser.
Conventional core pumping, in which pump light is coupled only into the small core, was initially used to achieve single mode output from a laser. However, a small core causes a serious restriction on pump power level. Furthermore, the core size leads to highly localized pump intensity which usually induces thermal damage at the fiber ends. To overcome this cladding pumping has been developed which provides the ability to divide the pump power launched into double clad fiber lasers. Both the core and the inner cladding layer are pumped. Some pump power is input to the core of the fiber, and some pump light is input into the inner cladding layer and propagates through it and gradually absorbs into the doped core within the cladding layer.
Absorption of pump power inside the core of a fiber laser causes heat generation which is highest at the launch end of the fiber laser and the heat generation reduces exponentially along the axis of the fiber laser moving away from the launch end. This high heat generation at the launch end of the fiber increases the potential for fiber failure and reduces the amount of pump energy that may be input to the launch end of a high power laser.
Alternative cladding shapes have given engineers a way to increase the total absorption of pump light per length of fiber without changing fiber composition. The shaped claddings prohibit helical cladding modes from propagating, and force all cladding modes on a path that intersect the doped core of the fiber laser near its launch end and increase the heat generated thereat. This results in significantly higher pump absorption coefficients for shaped claddings versus a classic round, cylindrical cladding. Mathematically this is introduced as a cladding mode scattering coefficient β, and is related directly to the shape of the fiber cladding. The cladding mode scattering coefficient β is highly dependent on the cladding shape and can vary drastically for the many different shapes of commercial optical fibers. While this approach increases the absorption it does not decrease the high heat generation at the launch end of the fiber laser.
Thus, there is a needed in the art for a new double clad fiber laser and a way to design the new double clad fiber laser to adjust how pump radiation is absorbed along the length of the fiber laser. This results in some of the thermal energy generated at the launch end of a double clad fiber being redistributed further down the fiber away from the launch end. The result of this is to reduce the thermal energy generated at the launch end of the fiber and allows for more pump radiation to be launched and subsequently absorbed and converted to a desired signal wavelength.